1. Field of the Invention
The present invention concerns a method for checking the positioning of an examination subject in a magnetic resonance system and a magnetic resonance system for implementing such a method. The invention in particular concerns a method for checking the positioning and a magnetic resonance system to achieve increased patient safety, by preventing possible injury to the patient by the formation of closed current loops in the patient due to the switching of magnetic field gradients.
2. Description of the Prior Art
Examinations using magnetic resonance tomography are used for various medical inquiries. Shorter examination times are always desired, not only for cost reasons but also in order to be able to obtain images of moving organs. For acquisition of MR images, radio-frequency pulses are used in connected with magnetic gradient fields that are switched on and off with very short switching times in order to shorten the acquisition time of an MR image as well as for contrast optimization.
Patient safety, however, must simultaneously be ensured. When radio-frequency pulses that are radiated into the body, a disadvantageous positioning of the examination subject (i.e. the patient) can cause the problem of closed current loops arising, that are induced in the body of the examination subject by the temporally varying magnetic flux. The induced voltage, and with it the current induced in the body, increase as the temporal change (rate of change) of the magnetic flux increases. The induced voltage and current also are dependent on the shape, position and size of the possible current loops. In the positioning of a patient before an MR examination, attention consequently must be paid to ensure that the positioning of the patient does not result in any geometric configurations that could lead to closed current loops. Among other things, such loops can result when the extremities contact one another or when a body extremity contacts another region of the body. It is consequently the task of the operating personnel of magnetic resonance systems to monitor the positioning of patients and to ensure that such closed current loops are prevented.
Closed current loops that can lead to burns on the examined patient do not always occur when the positioning of the patient is such that a current loop could form. Nevertheless, for the patient safety it must be ensured that the pre-condition for the formation of such current loops does not exist during the MR examination. Furthermore, recently new techniques have made it possible to acquire whole-body exposures by nuclear magnetic resonance. This is achieved, among other things, by the patient bed either being continuously moved with the patient thereupon, or shifted in steps through the magnetic resonance system, whereby respective MR images of the body part temporally located in the magnet are acquired and these are subsequently assembled.